In recent years, there has been a requirement to enhance the solutions for environmental pollution caused by exhaust gases from internal combustion engines. Since improvement of the ignition performance of spark plugs contributes to the purification of exhaust gas, there has been a spark plug provided with a noble metal member (tip) which has a high spark consumption resistance and protrudes from the inner surface of a ground electrode toward a center electrode (for example, refer to Patent Document 1). In the spark plug having this construction, since the ground electrode can be provided to be further away from the spark discharge gap as compared with an existing one, it is difficult for a flame kernel produced in the spark discharge gap to contact the ground electrode in an initial step of the growth process. Accordingly, the inhibition of growth caused by the heat loss of a flame kernel due to contact with the ground electrode, that is, a so-called flame-quenching, is alleviated, so that it is possible to improve the ignition performance of the spark plug.
In the spark plug having this construction, since a larger thermal load is exerted on the noble metal member, there is a concern that cracks or separation may occur in the junction portion between the noble metal member and the ground electrode. Therefore, in Patent Document 1, at the junction between the noble metal member and the ground electrode, a base part (intermediate member) having a linear expansion coefficient between those of the member and the electrode is interposed between the member and the electrode. The junction portion between the noble metal member and the base part tends to have the large thermal load. Thus, by reducing thermal stress that may occur on the junction portion, the generation of cracks and separation and the like is prevented. In Patent Document 1, in order to join the noble metal member and the base part to each other, resistance welding for exerting excessive pressing force during joining is not performed, but laser welding is performed in which heat concentration is easy, the fusing depth can be increase, and internal stress hardly remains after the joining.
[Patent Document 1] JP-A-2004-134209